The present invention relates generally to diode structures for integrated circuits and more specifically to an improved high voltage diode structure for integrated circuits.
The fabrication of high voltage diodes presents several problems. The doping of the more lightly doped side of the diode must be decreased to increase avalanche voltage. The resultilng higher resistivity leads to higher series resistance which is undesirable.
The planar structure is desired for its low leakage, ease of manufacture and reproducible characteristics. Field crowding around the edge of the top layer of a planar diode reduces breakdown voltage for a given background doping.
In diodes, a depletion layer in the lightly doped side exists which is proportional to (V/N) l/n where V is reverse voltage applied to the diode, N is doping concentration of the lightly doped side, and 2.ltoreq.n.ltoreq.3. At high voltages, V is large and N is small leading to large depletion layers. Such depletion layers require thick islands in integrated circuits which include these diodes to prevent the depletion from extending into the support substrate. Thick islands, as is well known, cause loss of packing density in IC's thus undesirably increasing cost.
One limit to high temperature operation of PN diodes arises from the loss of doping control of the lowest doped region of the diode when the temperature dependent intrinsic carrier concentration equals the impurity doping concentration. Above the temperature at which this equality is reached, diode characteristics degrade dramatically. The high resistivity regions used for conventional high voltage diodes thus lead to restrictive maximum operating temperature for these diodes.
The use of field plates at the surface of a junction to increase the breakdown voltage of the junction is a well known phenomenon. A typical example is U.S. Pat. No. 4,232,328 to Hartman et al. Hartman et al. also biases the polycrystalline support to produce a potential in the support layer to induce a charge of greater/lesser degree in the insulating layer underlying the pocket which attracts/repels the injected minority carries depending upon polarity. In a related patent, U.S. Pat. No. 4,242,697, to Berthold et al., a semi-insulating layer between the single crystalline tub and the polycrystalline substrate has trapping state capability of taking on charge from the single crystalline region. This prevents the potential from the substrate from causing the breakdown of the device and allows the surface regions of the device to remain closer to the polycrystalline substrate.
Thus, it is an object of the present invention to provide an improved diode structure which is capable of high voltage operation.
Another object of the present invention is to provide a high voltage diode structure which is less sensitive to temperature variations.
A still even further object of the present invention is to provide a diode structure having a substantially high background doping and built in a substantially minimal amount of surface space.
Still even a further object of the present invention is to provide a high voltage diode structure which is capable of being built in isolated substrates having a depth greater than that normally effected by the control of a surface field plate.
These and other objects of the invention are attained by using a field plate separated from the surface of the PN junction by an insulative layer and biasing the substrate which is also separated from the PN junction by a dielectric layer sufficient to deplete the entire portion of the background region there between before the critical field for avalanche is reached for that portion. The substrate is a conductive support structure and the diode is built in dielectrically isolated wells. Selecting the impurity concentration of the PN regions take within two orders of magnitude of each other which reduces the series resistance of the diode. The field plate and substrate are biased to voltage less than the more lightly doped region and preferably at the voltage of the more highly doped region.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.